Such a process is known from EP 0740002 B1. In this patent publication a process for making a fishing-line from yarns of filamentous materials is described, wherein a line made from braided, twisted, or twisted and plied yarns of gel spun polyolefin filaments is exposed to a temperature within the melting point range of said polyolefin for a time sufficient to at least partially fuse adjacent filaments while stretching said line at a stretching ratio within the range from 1.01 to 2.5, preferably from 1.35 to about 2.2. It is indicated that applying such stretch ratio to the precursor during the heat exposure is needed in order to keep the filaments under elongational tension, so as to prevent decrease of the strength of the product as a result of thermal molecular relaxation processes. The yarns applied in this process are continuous multi-filament yarns, more specifically such yarns made by so-called gel spinning of ultra-high molar mass polyethylene (UHMWPE), for example yarns commercially available under the trademarks Spectra® or Dyneema®. The monofilament-like products thus obtained in EP 0740002 B1 typically show a tenacity of from 13 to 32 g/d, and an elongation at break of from 1.9 to 3.3%.
Fishing lines are generally monofilaments made from synthetic polymers, having a round, firm structure that allows convenient handling for bait casting, spinning, and spin casting. Such monofilament lines generally have a stiff nature and smooth surface, which combine to reduce drag during the cast and enable longer casts while providing better release from fishing reels. Braided lines containing a multitude of filaments are less suited for fishing lines, because they have a tendency to fray at the end of the line, may entrap water, present an outer surface that is vulnerable to snags and entanglement, and have an opaque appearance that is too visible below water. The process known from EP 0740002 B1 allows making monofilament-like fishing lines from braided or twisted lines made from polyolefin multi-filaments yarns, which lines have specific advantages over braided lines. The performance of such fused lines also compares favourable with that of a conventional monofilament made from e.g. polyamide by melt extrusion in view of is higher tensile strength (or tenacity) and stiffness; but its elongation at break is significantly lower (about 2-3% versus 10-20%). On the one hand, low elongation and high modulus are advantageous for a fishing line, because it allows a fisherman to feel even an initial bite of a fish on a lure. On the other hand, a low elongation results in relatively low total energy absorption upon instant heavy loading, as upon hooking a fish, and may thus result in premature breaking. A line of low elongation, or low elasticity, also more readily injures biting fish. Therefore, it is desirable to have a monofilament-like product made from a precursor containing a multitude of continuous polyolefin filaments that combines higher elongation with comparable stiffness and strength as the known lines, especially the strength of a line containing a knot (knot strength).
There is thus a constant need for fishing lines offering improved performance, especially improved tensile properties. It is therefore an object of the present invention to provide a process for making a monofilament-like product with enhanced tensile properties.
This object is achieved according to the invention with a process for making a monofilament-like product from a precursor containing a multitude of continuous polyolefin filaments, comprising exposing the precursor to a temperature within the melting point range of the polyolefin for a time sufficient to at least partly fuse adjacent fibres while simultaneously stretching the precursor at a draw ratio of at least 2.7.
With the process according to the invention a monofilament-like product can be made from e.g. a plied or braided construction of polyolefin yarns, which product shows favourable tensile properties, such as a higher elongation at break, as measured in a tensile test as specified in ASTM D885M, more specifically by using a nominal gauge length of the fibre of 500 mm, a crosshead speed of 50%/min and Instron 2714 clamps. The monofilament-like product obtained by the process according to the invention typically shows an elongation at break of at least 4.0%, which makes it very suitable for use as fishing-line, as surgical suture and the like. The monofilament-like product obtained by the process according to the invention also shows high knot strength and knot strength efficiency. The mono-filament-like product obtained further has a pleasant touch or feel and can be easily handled and knotted. Another advantage of the process according to the invention is that it can be applied with high efficiency to twisted or air-entangled multifilament yarns, whereas in the known process braided precursors were applied for best results. The process according to the invention also offers more flexibility, in that a range of products with varying linear density (titer) can be made from one precursor. This means a simplification of the overall production process, and thus a more cost effective production.
It is true that applying a higher draw ratio during thermal fusion of a line containing polyolefin fibres is also suggested in WO 2004/033774 A1, but the process described therein is applied to a precursor containing spun yarn made from short staple fibres, which precursor has a completely different construction than a precursor containing continuous filaments. In addition, in the examples in WO 2004/033774 A1 only a draw ratio of at most 1.8 is disclosed; and elongation at break of a product made tends to decrease with increasing draw ratio.
It is further known that a relatively high draw ratio can be applied in the (post-)stretching steps forming part of the so-called gel spinning process of polyolefin into high-strength fibres; for example in EP 0205960 post-stretching is indicated to improve creep-resistance of multifilament UHMWPE yarn. Said publication, however, is silent on thermally fusing multifilament yarn into a monofilament-like product.
With the process according to the invention a monofilament-like product is made from a precursor containing a multitude of continuous polyolefin filaments. A monofilament-like product is understood to be a product that has an appearance and feel more resembling that of a monofilament than that of multi-filament yarn or cord, but which actually is made from a multitude of continuous filaments that typically have a diameter of less than about 50, often less than 30 micrometer. The monofilament-like product may have a diameter that varies within a wide range, e.g. from about 0.05 up to several millimeters (or more general products of titer from e.g. 10 up to several thousands dtex). A precursor is herein understood to be an article of indefinite length containing a multitude of continuous polyolefin filaments, for example one or more multifilament yarns of titre 50-2000 dtex, and is used as feed or starting material in the process according to the invention. A suitable precursor can be in the form of for example a braided cord, a plied and twisted yarn, cord or rope comprising a number of strands containing polyolefin filaments, but also a single-strand yarn. The precursor contains predominantly polyolefin filaments, i.e. 50 or more mass % of the total amount of filaments, preferably it contains at least 70, 80, 90 mass % of polyolefin filaments, or even substantially consists of only such filaments. This results in a line with high mechanical performance.